This invention relates to a method and apparatus for changing dies on a moving bolster for a press, particularly a large press. More particularly, this invention relates to a pneumatic control circuit adapted to decelerate the moving bolster to a smooth stop within or away from the press. Still more particularly, this invention relates to a slowdown valve for use in a circuit of the type described for controlling the movement of a moving bolster during die changing.
Large presses are well known for making large items such as automobile body parts by pressing a metal blank. Typically, such presses include a pressing apparatus for forming a part from a metallic blank which is inserted between the pressing apparatus and a die positioned in a bolster.
A typical arrangement utilizes a moving bolster, which is withdrawn from the press when a change of the die in the bolster is required. For example, the die for an automobile side panel may be removed from the bolster and another die, such as a hood die, is secured to the bolster in its place. Thereafter, the bolster with the hood die attached is moved into position and secured within the press. This procedure, and indeed the art of presses in general is well-developed.
However, the expeditious changing of dies remains a problem in this art. In addition to minimizing production interruption for a die change in normal circumstances, it is desired to minimize inventory of a particular part for cost reasons. Thus, while large numbers of a particular part might be pressed before changing to another part because of the long time needed to change the dies, now it is a significant desire in the art to produce fewer parts before a die change. This requires therefore that die changes be expedited and completed in a short time.
Such presses are massive with capacities up to thousands of tons of pressure to be exerted on dies which can weigh in the order of 40 or 50 tons. Accordingly, the bolsters and the dies are able to withstand such pressures in the pressing operation, largely through their size and structure. It is a problem, however, when undergoing expedited die changing of the type described, to move such massive parts in a way which satisfactorily controls movement and momentum of the combination, both in withdrawing the die/bolster combination from the press, but particularly in inserting the new die/bolster combination into the press. It is of course desirable to move the die and bolster as promptly as possible, but a risk of damage to the equipment for failure to stop adequately must be avoided given the masses involved.
Accordingly, it is a problem in this art to provide a control circuit, such as pneumatic control circuit compatible with typical control circuits for moving such equipment, which can control acceleration, deceleration, and stopping of such equipment.
It is another problem in this art to automate such stopping particularly within the press, since operator judgment if faulty can cause significant damage for failure of the equipment to stop precisely within the press. Thus, it is desired to provide a circuit which is responsive to a positive cam on the track of travel to initiate deceleration and stoppage.
It is another problem in this art to provide a control valve responsive to the cam for use in the pneumatic circuit of the invention to control deceleration and stoppage of the die and bolster at a predetermined time, with a desired hold and subsequent release of the pneumatic braking.
These and other objects of the invention will be apparent from a detailed description of the invention which follows.